Researchers have investigated various multi-color photovoltaic cells to increase the overall efficiency of photovoltaic cells and to extract the maximum amount of energy from solar radiation. These multi-color photovoltaic cells can be divided into two general categories. The first category covers monolithic multi-color photovoltaic cells. A monolithic multi-color photovoltaic cell is a photovoltaic cell which has distinct regions optimized to absorb different portions of the solar radiation spectrum in a single cell. U.S. Pat. Nos. 4,404,421 and 4,451,691, incorporated herein by reference for all purposes, describe suitable monolithic cells. Although these monolithic cells are attractive from a system and manufacturing point of view, they will require considerable materials research to bring them to commercialization.
A second approach involves tandem mechanically stacked two-color photovoltaic cells. These cells comprise independent photovoltaic cells which are optimized to the different portions of the solar spectrum and are mechanically and electrically interconnected. These tandem mechanically stacked two-color photovoltaic cells offer a shorter path to commercialization primarily because one of the cells can be an already developed cell, such as Si or GaAs. U.S. application Ser. No. 645,456 filed Aug. 28, 1984, incorporated herein by reference for all purposes, describes a suitable high-band gap photovoltaic cell. Examples of high-band gap photovoltaic cells are GaAsP or AlGaAs or GaAs or Si photovoltaic cells, and the like.
These mechanically stacked cells often fall into the category of photovoltaic cells known as concentrator photovoltaic cells. A concentrator photovoltaic cell is a high efficiency photovoltaic cell which utilizes some sort of focusing optics to concentrate solar radiation from a strength of one sun to many suns, i.e., on the order of 50 to 1000 or more suns. The concentration of the solar radiation permits the photovoltaic cells to produce a greater amount of electricity per unit area than lower efficiency flat plate photovoltaic cells. This makes them especially useful for space applications where weight is of great concern and in jobs which require maximum electrical output with a minimum amount of surface area. However, a drawback to concentrator photovoltaic cells is the problem associated with interconnecting the two mechanically stacked photovoltaic cells and dissipating the heat generated by the concentration of the solar radiation. Thus, it would be highly desirable to have a mechanically stacked apparatus which can interconnect two photovoltaic cells while minimizing the effects of heat generated by the concentrated solar radiation.
In conventional mechanical stack designs, in particular, those using thin top cells, the heat generated in the top cell must be transmitted through the transparent adhesive bonding the two cells together. This can lead to undesirably high cell temperatures. To avoid this difficulty, it would be highly desirable to have a package design wherein heat spreaders incorporated therein are used both for the bottom and top cells. A further advantage would be to incorporate a wafer for the top cell that it thick enough to conduct the heat laterally to the top heat spreader. A still further advantage would be to have a deisgn which isolates the cells so that the effects of thermal expansion are reduced or minimized.
In monolithic cell designs, the top and bottom cells must generally be current matched or the performance of the cell is limited by the cell having the lower current. Sine current matching different bandgap photovoltaic cells can sometimes be difficult, it would be desirable to have a package which permits voltage matching of the two cells. Voltage matching is beneficial because the voltages of the cells change very little with variations in solar spectrum or with the cell degradation with space radiation damage. Thus, it would also be highly desirable to have a package design which can dissipate the heat and permit the easy wiring of numerous mechanically stacked cells into a module wiring configuration for voltage matching instead of current matching.
Furthermore, it would be desirable to specify two component photovoltaic cell materials which will generate voltages which are simple multiples of each other where one of these photovoltaic cells is a well developed cell and the other can be rapidly developed to reach near its theoretical limit performance. Materials which can be rapidly developed are simple binary compounds with large optical absorption coefficients which have already been used as photodetectors.